This invention is in the field of sewing machine upper thread tension devices, more particularly, to thread tension devices wherein the thread tensioning discs are urged together by a leverage system having a pivot point, variable in order to adjust the force with which the tension discs are urged together.
There is a substantial body of prior art on upper thread tension devices, including the U.S. Pat. Nos. 3,557,731 of Casas-Robert and 3,190,249 of Gegauf, Jr. In particular, the latter patent of Gegauf, Jr. discloses a modular form of construction for a relatively complicated tension device of relatively large size.
There is also found in the prior art the U.S. Pat. Nos. 3,667,414 of Illes et al., and 3,366,083 of Ketterer et al., both of which refer to upper thread tension devices in which force is applied through a lever arrangement, and the force is made variable by a sliding fulcrum for the lever. In this particular prior art, the latter patent shows a relatively complicated arrangement for varying the fulcrum point of the lever, and the former patent discloses a somewhat less complex arrangement, but one utilizing considerably more space.
What is required is an upper thread tension device having none of the above drawbacks, i.e., an arrangement which is both simple and compact.